CN115459226B - Zero-sequence current differential protection setting method for multi-circuit transmission lines with series compensation based on series compensation linearization model - Google Patents

Abstract

The present invention proposes a zero-sequence current differential protection setting method for a multi-circuit line containing series compensation based on a series compensation linearization model, comprising the following steps: S1. Introducing a series compensation linearization model to describe the series compensation equivalent impedance under the action condition of a metal oxide voltage limiter; S2. Calculating the fault current and the series compensation equivalent impedance of the multi-circuit line containing series compensation through the series compensation linearization model; S3. Calculating the zero-sequence current differential protection sensitivity coefficient using the series compensation equivalent impedance; S4. Calculating the minimum sensitivity coefficient using the required series compensation equivalent impedance and the zero-sequence current differential protection sensitivity coefficient and calculating the extreme short-circuit current that causes the zero-sequence current differential protection to erroneously operate and refuse to operate; S5. Calculating the setting zero-sequence current differential protection ratio braking coefficient using the minimum sensitivity coefficient and calculating the action set current value using the required extreme short-circuit current; S6. Outputting the setting result of the zero-sequence current differential protection for the multi-circuit line containing series compensation. The present invention realizes the setting of zero-sequence current differential protection on multi-circuit lines, improves the protection sensitivity, and reduces the risk of erroneous operation and refusal of protection.

Description

Zero-sequence current differential protection setting method for multi-circuit transmission lines with series compensation based on series compensation linearization model

Technical Field

The present invention relates to the technical field of zero-sequence current differential protection setting, and more specifically, to a zero-sequence current differential protection setting method for a series-compensated multi-circuit line based on a series-compensated linearization model.

Background Art

With the rapid development of social economy, the power system grid is expanding and the power transmission power of the power grid is increasing. In actual power grid projects, multiple circuits are usually used to improve the transmission capacity, and series compensation devices (abbreviated as series compensation) are installed on this basis to reduce the inductive impedance of the transmission line, improve the power system transmission capacity limit, and enhance the safety and stability of the system.

In the high-voltage line where the series compensation is located, the zero-sequence current differential protection is usually used as the main protection under the line high-resistance grounding fault. Because the zero-sequence component is used as the action component, the protection has the characteristics of high sensitivity. At the same time, under the asymmetric high-resistance grounding fault, the line short-circuit current is small, usually smaller than the series compensation bypass current, so the series compensation impedance cannot be ignored, and the influence of the series compensation needs to be considered in the fault calculation and protection principle.

At present, in the lines considering series compensation operation, for the convenience of calculation, the series compensation is usually equivalent to a fixed capacitive reactance for fault calculation and protection setting. In this case, the equivalent impedance of the series compensation device MOV when it is turned on is not considered, resulting in inaccurate setting. At the same time, when there is an out-of-zone fault, the series compensation reduces the impedance of the protected line, resulting in an increase in the fault current, which may cause the protection to malfunction; when there is an in-zone fault, the system equivalent impedance is reduced due to the influence of multiple lines, and the series compensation will cause the current differential protection to decrease in sensitivity, which may cause the protection to refuse to operate. In the existing zero-sequence current differential protection setting algorithm, these issues are not considered in detail, and reasonable quantitative setting is not performed, which poses a risk of protection malfunction and refusal to operate.

Chinese patent announcement number CN103762561B discloses a method for setting DC differential protection for a high-voltage DC transmission system. The DC differential protection adopts a three-stage protection mode. The invention realizes the main protection of the converter grounding fault, avoiding the misoperation of the protection caused by the inconsistent transient characteristics of the measuring components during system disturbances, but fails to consider the equivalent impedance under the conduction of the series-compensated MOV and the misoperation and refusal of the protection caused by multiple series-compensated lines.

Summary of the invention

In view of the fact that the existing technical solutions fail to consider the equivalent impedance when the series compensated MOV is turned on and the protection false operation and refusal caused by multiple series compensated lines, the present invention discloses a zero-sequence current differential protection setting method for multiple series compensated lines based on a series compensation linearization model, which improves the protection sensitivity and prevents the protection false operation and refusal caused by series compensation.

To achieve the above technical objectives, the technical solution of the present invention is as follows:

A zero-sequence current differential protection setting method for a series-compensated multi-circuit line based on a series-compensated linearization model comprises the following steps:

S1. Introduce the series compensation linearization model to describe the series compensation equivalent impedance under the operation condition of the metal oxide voltage limiter;

S2. Calculate the fault current and equivalent impedance of multi-circuit lines containing series compensation through the series compensation linearization model;

S3. Calculate the sensitivity coefficient of zero-sequence current differential protection using the series compensation equivalent impedance obtained in S2;

S4. Calculate the minimum sensitivity coefficient and the extreme short-circuit current that causes the zero-sequence current differential protection to malfunction and refuse to operate by using the series compensation equivalent impedance obtained in S2 and the zero-sequence current differential protection sensitivity coefficient obtained in step S3;

S5. Calculate the setting zero-sequence current differential protection ratio braking coefficient using the minimum sensitivity coefficient obtained in S4 and calculate the action setting current value using the extreme short-circuit current obtained in S4;

S6. Output the setting result of zero-sequence current differential protection for multi-circuit lines with series compensation.

Here, a series compensation linearization model considering the action of series compensation MOV is introduced to describe the series compensation equivalent impedance under a specific fault current: in normal operation, the series compensation MOV is not conducting, and the series compensation external equivalent impedance is the rated capacitive reactance value, that is, Z _CM = -jX _C ; when a fault occurs, the line current rises, the series compensation MOV branch conducts to shunt and limit the voltage, and the series compensation device is protected. At this time, the series compensation external equivalent impedance can be regarded as a parallel form of the capacitor and the MOV nonlinear resistor. Scholar Goldsworthy has conducted a large number of experimental tests and equated the parallel model to a linear equivalent model, that is, when the fault current satisfies I _pu > 0.98, the series compensation device can be equivalent to a linear resistor R _CM and a variable capacitor X _CM in series. The expression of the series compensation equivalent impedance is Z _CM = R _CM -jX _CM , where

Where _XC is the capacitive reactance value under the rated working condition of the series compensation, the current ratio is defined as _Ipu = _Il / _Ipt , _Il is the current flowing through the series compensation device, and _Ipt is the protection current level, which can be set at 2 to 3 times the rated working current.

Preferably, the node impedance matrix method is used to calculate the short-circuit current and equivalent impedance of the single-phase high-resistance grounding fault of the multi-circuit line containing series compensation. The current flowing through the series compensation (between nodes i and j) at the kth iteration is set to I _Ck , so I _pu.k =I _Ck /I _pt , and the equivalent impedance of the series compensation Z _CM.k is calculated based on this, and the mutual admittance _Yij.k , _Yji.k of the line between nodes i and j and their respective self-admittances _Yii.k , _Yjj.k can be obtained.

Among them, _Yii.k-1 , _Yjj.k-1 , _ZCM.k-1 are the node self-admittance and series compensation equivalent impedance of the k-1th iteration, _Zij is the compensated line impedance, and the series compensation equivalent impedance _ZCM contained in it in the initial state is the rated capacitive reactance _ZC . The line network admittance matrix Y with a total number of nodes n is

The node impedance matrix Z can be obtained by inverse calculation through the network admittance matrix Y.

The fault current is calculated using the node impedance matrix method. The calculation formula is as follows

in is the normal voltage at the fault point before short circuit, Z _ff is the self-impedance of the fault node f, and Z _f is the transition resistance.

The obtained series compensation current is subtracted from the fault current I _Ck-1 calculated in the k-1th iteration, and the series compensation current I _Ck is iteratively corrected.

The iteration ends when the convergence criterion is met.

max{|ΔI _k |}<ε

Wherein, ε is a given small positive number.

The calculated series compensation fault current is the series compensation line fault current I _pu.g =I _Cg /I _pt under the actual series compensation linearization model. The series compensation equivalent impedance obtained under actual working conditions is Z _CM.g , where g is the total number of final iterations.

Preferably, the sensitivity coefficient of zero-sequence current differential protection of multi-circuit lines with series compensation under single-phase high-resistance grounding fault is calculated. The ratio-restrained zero-sequence current differential protection criterion is:

Define the protection sensitivity coefficient K _sen0 as

The differential current is The braking current is The zero-sequence short-circuit currents on both sides of the series-compensated line are The zero-sequence differential action setting current is I _Th0 , and the zero-sequence differential ratio braking coefficient is k ₀ . Usually, I _Th0 is set according to the zero-sequence current level of the network. Calculate the sensitivity coefficient when there is a fault in the area containing multiple series-compensated lines. If K _sen0 <k ₀ , the protection fails to operate, and the protection ratio braking coefficient needs to be adjusted; if K _sen0 >k ₀ , the protection has a high sensitivity and can maintain the original value.

In a multi-circuit (N-circuit) series-compensated line, the sensitivity coefficient of zero-sequence current differential protection is

Among them, _Kd0 =1/[(N-1)( _ZSM0 + _ZSN0 )/( _ZL0 + _Z′CM )+1]<1 is satisfied, k is the fault location, _ZL0 is the line zero-sequence impedance, _ZSM0 and _ZSN0 are the system zero-sequence impedances on both sides, _ZCM is the series compensation equivalent impedance related to the fault line and the short-circuit current, and _Z′CM is the series compensation equivalent impedance on the other N-1 parallel lines.

Preferably, the extreme short-circuit current and minimum sensitivity coefficient of the series-compensated line are calculated. The maximum short-circuit current flowing through the line under the short-circuit fault outside the protected multi-circuit line is calculated to measure the unbalanced current I _unb0 of the differential protection for the fault outside the zone

in, is the maximum short-circuit current of three-phase metallic grounding of adjacent fault lines, and _Cd is the shunt coefficient of multiple circuits. If the differential current is greater than the protection setting value I _Th0 , the original protection setting value needs to be adjusted to avoid false protection operation.

The minimum sensitivity coefficient of the multi-circuit line protection with series compensation is that the systems on both sides work in the maximum operation mode, that is, the system impedance is minimum. At the fault position k _n = 0, the minimum sensitivity coefficient K _sen0.min under the single-phase high-resistance grounding condition can be obtained.

Among them, K _d0.m is the value of K _d0 under the maximum operation mode, is the impedance value of Z _CM at the fault location. If the minimum sensitivity coefficient is less than the protection setting k ₀ , the original protection setting needs to be adjusted to avoid protection failure.

Preferably, the zero-sequence current differential protection ratio braking coefficient and the action set current value are set. The zero-sequence current differential protection action set current is set, taking into account the zero-sequence unbalanced current and the steady-state zero-sequence distributed capacitance current under the fault condition outside the area of the series-compensated multi-circuit line.

Among them, _Krel is the reliability coefficient, _Ka is the non-periodic component coefficient, _Kst is the transformer isotype coefficient, _ft is the transformer amplitude error, It is the maximum fault current of the compensated line when there is a three-phase short-circuit fault outside the zone. In order to avoid false operation of the differential protection during normal operation, the set action value in actual engineering should be greater than the minimum set current I _0.th .

The zero-sequence current differential protection ratio braking coefficient is set, considering the most severe operating conditions of the systems on both sides. To achieve unified setting of multiple lines, the system impedance on both sides of the fault line is ignored, and the setting is based on avoiding the minimum protection sensitivity coefficient and the minimum braking coefficient. In the setting calculation, the system impedance on both sides of the fault line and the influence of multiple lines are ignored, and the minimum sensitivity coefficient of the high-resistance grounding protection at the series compensation outlet is taken as

Among them, Z _L0 is the zero-sequence impedance of the protected line, and Z _CM.n is the equivalent impedance of series compensation under the condition of maximum transition resistance at the fault point. The setting value k ₀ of the ratio restraint coefficient of zero-sequence current differential protection is set to

Among them, K _rel is defined as the reliability coefficient, k _0.th is the minimum braking coefficient to avoid false operation of protection when a three-phase short circuit fault occurs outside the fault line area, which can usually be set according to the actual grid conditions.

Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

The present invention proposes a zero-sequence current differential protection setting method for multi-circuit lines containing series compensation based on a series compensation linearization model, taking into account the influence of series compensation MOV action, and is suitable for zero-sequence current differential protection setting of multi-circuit lines, thereby improving protection sensitivity, preventing protection misoperation and refusal to operate due to series compensation, and realizing quantitative reasonable setting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart showing a method for setting zero-sequence current differential protection for multi-circuit lines containing series compensation based on a series compensation linearization model proposed by the present invention;

FIG2 is a schematic diagram showing the structure of a typical series capacitor compensation device provided in this embodiment;

FIG3 is a simplified schematic diagram of a series capacitor compensation device provided in an embodiment of the present invention;

FIG4 is a schematic diagram of a series compensation linearization model provided in an embodiment of the present invention;

FIG5 shows a zero-sequence network diagram of a fault in a series-compensated multi-circuit line area according to an embodiment of the present invention;

FIG6 shows a fault current distribution diagram outside the area of a series-compensated multi-circuit line according to an embodiment of the present invention.

DETAILED DESCRIPTION

The drawings and embodiments are only for illustrative purposes and cannot be understood as limiting the scope of application of this patent;

For those skilled in the art, descriptions of certain well-known contents in the drawings may be omitted.

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Example 1

As shown in FIG1 , a flow chart of a method for setting zero-sequence current differential protection of a multi-circuit line with series compensation based on a series compensation linearization model is shown. The method comprises the following steps:

S1. Introduce the series compensation linearization model to describe the series compensation equivalent impedance under the operation condition of metal oxide varistor (MOV);

The structure of the series compensation device is shown in Figure 2. In the specific implementation, a series compensation linearization model considering the action of the series compensation MOV is introduced to describe the series compensation equivalent impedance under a specific fault current: in normal operation, the series compensation MOV is not conducting, and the series compensation external equivalent impedance is the rated capacitive reactance value, that is, Z _CM = -jX _C ; when a fault occurs, the line current rises, and the series compensation MOV branch is conducting to shunt and limit the voltage to protect the series compensation device. At this time, the series compensation external equivalent impedance can be regarded as a parallel form of a capacitor and a nonlinear resistor of the MOV as shown in Figures 3 and 4. Scholar Goldsworthy has conducted a large number of experimental tests and equated the parallel model to a linear equivalent model, that is, when the fault current satisfies I _pu > 0.98, the series compensation device can be equivalent to a linear resistor R _CM and a variable capacitor X _CM in series. The expression of the series compensation equivalent impedance is Z _CM = R _CM -jX _CM , where

Where _XC is the capacitive reactance value under the rated working condition of the series compensation, the current ratio is defined as _Ipu = _Il / _Ipt , _Il is the current flowing through the series compensation device, and _Ipt is the protection current level, which can be set at 2 to 3 times the rated working current.

When the short-circuit current is greater than the MOV conduction level and the series compensation bypass condition is not met, the external equivalent impedance of the series compensation is Z _CM ; when the short-circuit current exceeds the maximum bypass current, the series compensation exits the operating state and the external equivalent impedance is 0.

S2. Calculation of fault current and equivalent impedance of series compensation multi-circuit lines;

The series compensation linearization model is introduced into the short-circuit calculation to obtain the short-circuit current and the series compensation equivalent impedance under specific fault conditions. The specific steps include:

S2.1: Use the node impedance matrix method to calculate the short-circuit current and equivalent impedance of the single-phase high-resistance grounding fault of the multi-circuit line with series compensation. Set the current flowing between the i and j nodes of the series compensation at the kth iteration to I _Ck , so I _pu.k = I _Ck /I _pt , and use this to calculate the equivalent impedance of the series compensation Z _CM.k , and the mutual admittance _Yij.k , _Yji.k of the line between the i and j nodes and their respective self-admittances _Yii.k , _Yjj.k can be obtained.

Among them, _Yii.k-1 , _Yjj.k-1 , _ZCM.k-1 are the node self-admittance and series compensation equivalent impedance of the k-1th iteration, _Zij is the compensated line impedance, and the series compensation equivalent impedance _ZCM contained in it in the initial state is the rated capacitive reactance _ZC . The line network admittance matrix Y with a total number of nodes n is

The node impedance matrix Z can be obtained by inverse calculation through the network admittance matrix Y.

S2.2: Calculate the fault current using the node impedance matrix method, including three-phase ground short circuit and single-phase ground short circuit calculation. Take the phase distance protection characteristics of the embodiment as an example, and perform three-phase short circuit calculation on the fault network. Then the node voltage V _i of node i under the fault condition and the branch current I _rm between nodes r and m under the fault condition are respectively

Wherein, node f is the fault node, V _i ⁽⁰⁾ is the node voltage of node i under normal working conditions, is the node voltage of node f under normal working conditions, z _f is the fault transition impedance, Z _if is the mutual impedance between nodes i and f, Z _ff is the self-impedance of node f, V _r , V _m , z _rm are the node voltages and line impedances at both ends of line rm, and k _t is the line voltage transformer ratio. If series compensation is installed between lines rm, the fault current is the series compensation current. If the embodiment is a single-phase high-resistance grounding fault, the series compensation impedance in the three-sequence impedance network is replaced by a series compensation equivalent model, and the fault current of the line containing series compensation is calculated, so it is not repeated.

The obtained series compensation current is subtracted from the fault current I _Ck-1 calculated in the k-1th iteration, and the series compensation current I _Ck is iteratively corrected.

The iteration ends when the convergence criterion is met.

max{|ΔI _k |}<ε

Wherein, ε is a given small positive number.

S2.3: The calculated series compensation fault current is the series compensation line fault current I _pu.g =I _Cg /I _pt under the actual series compensation linearization model. The series compensation equivalent impedance obtained under the actual working condition is Z _CM.g , where g is the total number of final iterations.

S3. Calculation of sensitivity coefficient of zero-sequence current differential protection;

The sensitivity coefficient of zero-sequence current differential protection of multi-circuit lines with series compensation under single-phase high-resistance grounding fault is calculated. The criterion of ratio-restrained zero-sequence current differential protection is:

Define the protection sensitivity coefficient K _sen0 as

The differential current is The braking current is As shown in Figure 5, the zero-sequence short-circuit currents on both sides of the series-compensated line are The zero-sequence differential action setting current is I _Th0 , and the zero-sequence differential ratio braking coefficient is k ₀ . Usually, I _Th0 takes a specific value according to the zero-sequence current level of the network, and k ₀ takes a constant of 0.75. Calculate the sensitivity coefficient when there is a fault in the area containing multiple series-compensated lines. If K _sen0 <k ₀ , the protection refuses to operate, and the protection ratio braking coefficient needs to be adjusted; if K _sen0 >k ₀ , the protection has a high sensitivity and can maintain the original value.

Example 2

As shown in the embodiment of FIG5 , the impedance of the multi-circuit (N-circuit) line is equivalent, and in the fault line, the zero-sequence current differential protection on both sides measures the current It can be expressed as

Then the sensitivity coefficient of zero-sequence current differential protection is

Among them, K _d0 =1/[(N-1)(Z _SM0 +Z _SN0 )/(Z _L0 +Z′ _CM )+1]<1 is satisfied, is the zero-sequence current at the fault point, k is the fault location, Z _L0 is the line zero-sequence impedance, Z _CM is the series compensation equivalent impedance related to the fault line and the short-circuit current, Z _SM0 and Z _SN0 are the system zero-sequence impedances on both sides, Z′ _CM is the series compensation equivalent impedance on the other N-1 parallel lines, and Z ₁ and Z ₂ are the equivalent calculated impedances of multiple lines obtained according to Y-Δ transformation.

S4. Calculation of the extreme short-circuit current and minimum sensitivity coefficient that causes the zero-sequence differential protection to malfunction and fail to operate;

S4.1: Calculation of extreme short-circuit current of series-compensated lines. As shown in the embodiment of FIG6 , the maximum short-circuit current flowing through the protected multi-circuit lines under the short-circuit fault outside the area is calculated to measure the size of the unbalanced current I _unb0 of the differential protection of the fault outside the area

in, is the maximum short-circuit current of three-phase metallic grounding of adjacent fault lines, and C _d (<1) is the shunt coefficient of multiple circuits. If the differential current is greater than the protection setting value I _Th0 , the original protection setting value needs to be adjusted to avoid false protection operation.

S4.2: Calculation of minimum sensitivity coefficient of series compensation line protection. The minimum sensitivity coefficient of series compensation multi-circuit line protection is that the systems on both sides work in the maximum operation mode, that is, the system impedance is minimum. At the fault position k _n = 0, the minimum sensitivity coefficient K _sen0.min under the single-phase high-resistance grounding condition can be obtained.

Among them, K _d0.m is the value of K _d0 under the maximum operation mode, is the impedance value of Z _CM at the fault location. If the minimum sensitivity coefficient is less than the protection setting k ₀ , the original protection setting needs to be adjusted to avoid protection failure.

Example 3

In this embodiment, in step S5, the method for setting the zero-sequence current differential protection ratio braking coefficient and the action fixed current value is as follows:

S5.1: Set the zero-sequence current differential protection action setting current, considering the zero-sequence unbalanced current and steady-state zero-sequence distributed capacitance current under the fault condition of multi-circuit lines with series compensation outside the area

Among them, the reliability coefficient K _rel is 1.3, the non-periodic component coefficient _Ka is 1.5-2.0, the transformer isomorphic coefficient _Kst is 0.5, and the transformer amplitude error _ft is 0.1. It is the maximum fault current of the compensated line when there is a three-phase short-circuit fault outside the zone. In order to avoid false operation of the differential protection during normal operation, the set action value in actual engineering should be greater than the minimum set current I _0.th . Usually, it can be taken as 250A in engineering.

S5.2: The zero-sequence current differential protection ratio braking coefficient is set, considering the most severe operating conditions of the systems on both sides. To achieve unified setting of multiple lines, the system impedance on both sides of the fault line is ignored, and the setting is performed according to the minimum protection sensitivity coefficient and the minimum braking coefficient. In the setting calculation, the system impedance on both sides of the fault line and the influence of multiple lines are ignored, and the minimum sensitivity coefficient of the high-resistance grounding protection at the series compensation outlet is taken as

Among them, Z _L0 is the zero-sequence impedance of the protected line, and Z _CM.n is the equivalent impedance of series compensation under the condition of maximum transition resistance at the fault point. The setting value k ₀ of the ratio restraint coefficient of zero-sequence current differential protection is set to

Among them, K _rel is defined as the reliability coefficient, k _0.th is the minimum braking coefficient to avoid false protection operation when a three-phase short circuit fault occurs outside the fault line area, which can usually be set according to the actual grid conditions and can be taken as 0.6. If the braking coefficient value is not less than 0.75, the initial setting value of 0.75 will be maintained.

S6. Output the setting result of zero-sequence current differential protection for multi-circuit lines with series compensation, that is, the zero-sequence current differential protection action setting value.

Claims (6)

1. A series compensation multi-circuit line zero sequence current differential protection setting method based on a series compensation linearization model,

The method is characterized by comprising the following steps of:

s1, introducing a series compensation linearization model to describe series compensation equivalent impedance under the action working condition of a metal oxide voltage limiter;

s2, calculating fault current and series compensation equivalent impedance of the multi-circuit line with series compensation through a series compensation linearization model;

s3, calculating a zero sequence current differential protection sensitivity coefficient by utilizing the serial compensation equivalent impedance obtained in the S2;

S4, calculating a minimum sensitivity coefficient by utilizing the serial compensation equivalent impedance obtained in the S2 and the zero sequence current differential protection sensitivity coefficient obtained in the step S3, and calculating an extreme short-circuit current causing zero sequence current differential protection malfunction rejection;

s5, calculating a set zero sequence current differential protection ratio braking coefficient by using the minimum sensitivity coefficient obtained in the S4, and calculating an action fixed value current value by using the extreme short circuit current obtained in the S4;

S6, outputting a setting result of zero sequence current differential protection of the serial compensation multi-circuit line;

The serial compensation device under the action working condition of the metal oxide voltage limiter is equivalent by adopting a serial compensation linearization model, the equivalent is a serial impedance structure of a linear resistor R _CM and a variable capacitor X _CM, the equivalent impedance is Z _CM＝R_CM-jX_CM, wherein

Wherein X _C is the capacitance value under the serial compensation rated working condition, the current multiplying power is defined as I _pu＝I_l/I_pt,I_l which is the current flowing through the serial compensation device, and I _pt is the protection current level;

Introducing a series compensation linearization model into multi-circuit line short circuit calculation to obtain series compensation equivalent impedance under the specific fault working condition and series compensation multi-circuit line fault current-containing series compensation multi-circuit line fault current, wherein the specific step S2 comprises the following steps:

s2.1: describing impedance values of the serial compensation corresponding to the actual current change by using the impedance expression of the serial compensation linearization model in the step S1, substituting the impedance values into network data to generate a fault calculation node impedance matrix, and setting the current flowing between the serial compensation I and j nodes as I _C.k in the calculation process in the following way when the k iteration is performed, so that I _pu.k＝I_C.k/I_pt is used for calculating serial compensation equivalent impedance Z _CM.k, and obtaining line admittance Y _ij.k、Y_ji.k between the I and j nodes and self admittance Y _ii.k、Y_jj.k of each line admittance

Wherein Y _ii.k-1、Y_jj.k-1、Z_CM.k-1 is the k-1 iteration node self admittance and series compensation equivalent impedance, Z _ij is the compensated line impedance, the series compensation equivalent impedance Z _CM contained in the initial state is the rated capacitance Z _C, and the line network admittance matrix Y with the total node number of n is

The node impedance matrix Z can be obtained through inversion calculation through the network admittance matrix Y;

S2.2: the node impedance matrix method is used for calculating the fault current, and the calculation formula is as follows

Wherein the method comprises the steps ofAs the normal voltage of the fault point before short circuit, Z _ff is the self-impedance of the fault node f, and Z _f is the transition resistance;

Iterative correction of the previous series compensation current I _C.k-1 is carried out by adopting the calculated series compensation fault current I _C.k

S2.3: if the iteration correction meets the iteration requirement

max{ΔI_k|}<ε

Wherein ε is a given small positive number;

obtaining fault current of the multi-circuit line containing series compensation, substituting the fault current into the series compensation equivalent calculated impedance of the series compensation linearization model impedance expression calculated in the step S1; otherwise, returning to the step S2.2;

according to the differential protection criterion of the ratio brake type zero sequence current

Defining the protection sensitivity coefficient K _sen0 as

Wherein, the differential current is I _diff0, the braking current is I _bias0, the zero sequence differential protection action constant value current is I _Th0, the zero sequence differential ratio braking coefficient is k ₀,I_Th0, and the zero sequence current differential protection constant value is set according to the network zero sequence current level;

the method for calculating the minimum sensitivity coefficient of the zero sequence differential protection misoperation-causing extreme short circuit current and protection refusal operation under the multi-circuit line comprises the following specific steps of:

S4.1: calculating the maximum short-circuit current flowing through the line under the three-phase metallic grounding short-circuit fault outside the protected multi-circuit line area by using a node impedance matrix method, and measuring the unbalanced current I _unb0 of the differential protection of the fault outside the area;

Wherein, For the maximum short-circuit current of the three-phase metallic grounding of the adjacent fault line, C _d is the shunt coefficient of the multi-circuit line, if the differential current I _diff0 is judged to be larger than the protection fixed value I _Th0, the original protection fixed value is required to be set and modified to avoid protection misoperation;

S4.2: calculating minimum sensitivity coefficient K of zero sequence current differential protection under single-phase high-resistance grounding working condition in multi-circuit line region containing series compensation _sen0.min

Wherein, K _d0.m is the value of K _d0 in the maximum operation mode,An impedance value of Z _CM at the fault location;

Aiming at the working condition of the series compensation multi-circuit, the setting value calculation with higher quantification and adaptability is respectively carried out on the zero-sequence current differential protection ratio brake coefficient and the action constant current, and the specific step S5 comprises the following steps:

s5.1: setting the constant value current of the differential protection action of the zero sequence current, and considering the zero sequence unbalanced current and the steady-state zero sequence distributed capacitance current under the fault condition of the out-of-zone fault of the multi-circuit line containing series compensation

Wherein, K _rel is a reliability coefficient, K _a is an aperiodic component coefficient, K _st is a transformer model coefficient, f _t is a transformer amplitude error,The maximum fault current of the compensated line during the out-of-zone three-phase short circuit fault is the maximum fault current, and in order to avoid the false action of differential protection during normal operation, the setting action fixed value in the actual engineering should be larger than the minimum fixed value current I _0.th;

S5.2: setting the braking coefficient of the differential protection ratio of the zero sequence current, considering the most severe operating conditions of the systems at two sides, and setting the braking coefficient setting value k ₀ of the differential protection ratio of the zero sequence current to be the setting value k ₀ for realizing the unified setting of the multi-circuit line and neglecting the system impedance at two sides of the fault line

K _rel is defined as a reliable coefficient, K _0.th is a minimum braking coefficient for avoiding protection misoperation when a three-phase short circuit outside a fault line area is in fault, and the minimum braking coefficient is set according to the working condition of an actual power grid.

2. The differential protection setting method for zero sequence current of multi-circuit lines with serial compensation based on the serial compensation linearization model according to claim 1, wherein the protection current level I _pt is 2-3 times of the rated working current.

3. The method for setting zero sequence current differential protection of a multi-circuit line containing serial compensation based on a serial compensation linearization model according to claim 1, wherein if K _sen0<k₀ occurs, the protection is refused, and the protection ratio braking coefficient is set; if K _sen0>k₀ is adopted, the protection has higher sensitivity, and the original fixed value operation is kept.

4. The differential protection setting method for zero sequence current of multi-circuit lines with serial compensation based on serial compensation linearization model according to claim 3, wherein k ₀ takes a constant of 0.75.

5. The method for setting zero sequence current differential protection of multiple circuit lines including series compensation based on series compensation linearization model as set forth in claim 1, wherein if the minimum sensitivity coefficient is less than the protection fixed value k ₀, the original protection fixed value is set and modified to avoid protection refusal.

6. The method for setting zero sequence current differential protection of a multi-circuit line with serial compensation based on a serial compensation linearization model according to any one of claims 1 to 5, wherein a reliability coefficient K _rel is 1.3, an aperiodic component coefficient K _a is 1.5 to 2.0, a transformer homotype coefficient K _st is 0.5, and a transformer amplitude error f _t is 0.1; if the brake coefficient value is not less than 0.75, the initial setting value of 0.75 will be maintained.